1. Technical Field
The present disclosure relates to a topological insulator structure.
2. Discussion of Related Art
If an electric current flows through an electrical conductor in a magnetic field perpendicular to the electric current, a measurable voltage difference between two sides of the electrical conductor, transverses to the electric current and the magnetic field, will be produced. The presence of this measurable voltage difference is called the Hall effect (HE) discovered by E. H. Hall in 1879. Subsequently, the anomalous Hall effect (AHE) in magnetic materials and the spin Hall effect (SHE) in semiconductors were discovered. Theoretically, HE, AHE, and SHE would have corresponding quantized forms. In 1980, K. V. Klitzing et al. achieved quantum Hall effect (QHE) in a semiconductor in a strong magnetic field at a low temperature (Klitzing K. V. et al., New Method for High-Accuracy Determination of the Fine-Structure Constant Based on Quantized Hall Resistance, Phys Rev Lett, 1980, 45:494-497). After that, D. C. Tsui et al. achieved fractional quantum Hall effect (FQHE) during the studying of the HE in a stronger magnetic field (Tsui D. C. et al., Two-Dimensional Magnetotransport in the Extreme Quantum Limit. Phys Rev Lett, 1982, 48:1559-1562). In 2006, Shoucheng Zhang predicted that quantum spin Hall effect (QSHE) can be realized in mercury telluride-cadmium telluride semiconductor quantum wells (Bernevig B. A. et al., Quantum spin Hall effect and topological phase transition in HgTe quantum wells, Science, 2006, 314:1757-1761). This prediction was confirmed in 2007 (Konig M. et al. Qauntum spin Hall insulator state in HgTe quantum wells. Science, 2007, 318:766-770). At present, in the variety of quantized forms of the HE, only the quantum anomalous Hall effect (QAHE) has not been observed in reality. QAHE is the QHE in zero magnetic field without Landau levels, which can have a Hall resistance of h/e2 (i.e., 25.8 kΩ, i.e., quantum resistance), wherein e is the charge of an electron and h is Planck's constant. The realizing of the QAHE can get rid of the requirement for the external magnetic field and the high electron mobility of the sample, and has an application potential in real devices.
Topological insulators (TIs) are a class of new concept quantum materials. A TI has its bulk band gapped at Fermi level, the same as usual insulators, but hosts gapless, Dirac-type, and spin-split surface states at all of its surfaces, which allow the surfaces to be electrically conductive and are protected by time reversal symmetry (TRS). There are two kinds of TIs, three-dimensional (3D) TIs and two-dimensional (2D) TIs. 3D TIs have topologically-protected two dimensional surface states. 2D TIs have topologically-protected one dimensional edge states. The discovery of Bi2Se3 group (including Bi2Se3, Bi2Te3, and Sb2Te3) of TIs makes this kind of material receives substantial research interest from not only condensed matter physics but also material science. In 2010, Yu R. et al. predicted that QAHE could be achieved in Cr or Fe doped Bi2Se3, Bi2Te3, and Sb2Te3 3D TI films (Yu R. et al., Quantized anomalous Hall effect in magnetic topological insulators, Science, 2010, 329:61-64). However, any TI which can observe the QAHE therein has not been achieved. Further, even a ferromagnetic material (including magnetic doped TIs) having an anomalous Hall resistance larger than a kiloohm (kΩ) has not been achieved. For a film having a thickness of 5 nanometers and having the anomalous Hall resistance larger than a kiloohm, the corresponding anomalous Hall resistivity should be larger than or equal to 0.5 milliohms·millimeter (mΩ·mm).
What is needed, therefore, is to provide a TI structure having a relatively large anomalous Hall resistance.